1. Field of the Invention
The present invention relates to a technique to suppress an offset of toner in a fixing unit that is mounted to an image forming apparatus.
2. Description of the Related Art
A fixing unit in an image forming apparatus applies heat to a toner image and a sheet in order to fix the toner image that is transferred to the sheet. Although ceramic heaters and halogen heaters were mainly used as a heat source, an electromagnetic induction heating method has come to be utilized in recent years. The electromagnetic induction heating method is a method to let the fixing roller generate heat by generating an eddy current in a fixing roller with an electromagnetic induction coil.
Incidentally, when a sheet passes the nip of the fixing unit, the surface of the fixing roller is charged by friction of the sheet with the fixing roller and a pressing roller. Meanwhile, the toner on the sheet that arrives at the fixing unit is charged by an image forming process. When the polarity of the surface of the fixing roller and the polarity of the toner are opposite from each other, or the polarity of the surface of the pressing roller and the polarity of the toner are the same, a so-called offset occurs, which is a phenomenon that the toner on the sheet adheres to the fixing roller.
In Japanese Patent No. 4040348, a bias circuit is proposed, which is configured to prevent the offset in a fixing unit in the electromagnetic induction heating method. This bias circuit collects electric charge by a collector member contacting a magnetic core, stores the collected electric charge in an external rectification circuit and a charging capacitor, and applies the stored electric charge to an electrically conductive layer of the fixing roller.
The bias circuit described in the Japanese Patent No. 4040348 has the advantage that it is capable to prevent the offset without providing a high voltage power supply for applying bias. However, in this bias circuit, since a collector member and a charging capacitor are required, a new problem has arisen, namely that it tends to increase the size and cost of the fixing unit.